‘Watermarking’ means imperceptible insertion of information into multimedia data, e.g. audio data and/or video data. The insertion of additional information data, such as a number or a text, into multimedia data is performed through slight modification of the original multimedia data. Watermarking can be used for e.g. copyright protection, labelling (e.g. URL of a site or a site's logo), monitoring, tamper proofing, or conditional access.
Applying ‘spread spectrum’ in a (RF) communications system, means that a small baseband signal bandwidth is intentionally spread over a larger bandwidth by injecting or adding a higher-frequency signal, or spreading function. As a direct consequence, the energy used for transmitting the signal is spread over a wider bandwidth, and appears as noise.
Spread spectrum technology and the related inserted or added information signal can be used for implementing watermarking of e.g. digital audio signals, whereby the spread spectrum can use the complete audio spectrum from 0 Hz to one half of the sampling frequency. This spectrum carries the information of one bit.
In a modification of such systems shorter spread spectrum sequences are used leading to band limited spread spectrum signals, so that several ones of the band limited spread spectrum signals can be added at different centre frequencies to the audio spectrum, at which centre frequencies the original audio signal has been notch filtered, in order to increase the bitrate of the watermark signals and/or to prevent attacks on the watermarked signals. In this watermark system the spread spectrum signals are modulated on a carrier.
The watermark signals that are embedded in the audio signal should not be audible. But if the original audio signal spectrum has no sufficient energy level near a modulation frequency, the watermark signal will become audible. Therefore the watermark signals are inserted only in those frequency bands where the audio signal has substantial power, based on psycho-acoustic laws.
A known processing for retrieving at receiver or decoder side the watermark signal information bit from the spread spectrum is convolving the received or replayed spectrum with a spreading function or sequence that is time-inverse with respect to the original spreading function or sequence, which kind of processing is also called ‘applying a matched filter’. If BPSK modulation was used for applying the spread spectrum function, the output of this process is a peak at the middle of the sequence of correlation overlap-add values, whereby the sign of such peak represents the value of the desired watermark signal information bit. If QPSK was used two peaks will be present in the sequence of correlation values, whereby each peak represents one bit value.